The invention relates to a dilution refrigerator assembly.
Dilution refrigerators are used for achieving ultra low temperatures for experiments in the millikelvin temperature range. A typical dilution refrigerator includes a still, a mixing chamber, and a heat exchanger connected between the still and mixing chamber whereby coolant flows from the still to the mixing chamber and from the mixing chamber to the still through respective first and second adjacent paths in the heat exchanger. Examples of known dilution refrigerators are described in U.S. Pat. No. 5,189,880, U.S. Pat. No. 5,542,256 and xe2x80x9cA Simple Dilution Refrigeratorxe2x80x9d by J. L. Levine, The Review of Scientific Instruments, Vol. 43, Number 2, February 1972, pages 274-277.
Typically, such a dilution refrigerator uses 3He/4He and makes use of the fact that when a mixture of these two stable isotopes of helium is cooled below its tri-critical temperature, it separates into two phases. The lighter xe2x80x9cconcentrated phasexe2x80x9d is rich in 3He and the heavier xe2x80x9cdilute phasexe2x80x9d is rich in 4He. Since the enthalpy of the 3He in the two phases is different, it is possible to obtain cooling by xe2x80x9cevaporatingxe2x80x9d the 3He from the concentrated phase into the dilute phase.
In order for dilution refrigerators to be used to investigate samples in high magnetic fields, it has been known to provide an elongate, tubular extension to the mixing chamber which extends into the bore of a magnet. In this case, it is necessary for the 3He return tube also to extend into the mixing chamber extension to promote circulation of 3He around the sample which in turn is held on the end of a holder extending through the refrigerator and the return tube. An example of such a dilution refrigerator which enables a sample to be xe2x80x9ctop-loadedxe2x80x9d is described in xe2x80x9cNovel Top-Loading 20 mK/15T Cryomagnetic Systemxe2x80x9d by P. H. P. Reinders et al, Cryogenics 1987 Vol. 27 December, pages 689-692.
Although these known dilution refrigerator assemblies work well, they are relatively inflexible and typically can only be used for one class of experiments at a time. In addition, properties such as the thermodynamic performance cannot be routinely upgraded.
In accordance with the present invention, a dilution refrigerator assembly comprises a first module including a dilution refrigerator; and a second module including experimental services for attachment to a sample located in use outside the dilution refrigerator, wherein the second module can be attached to and demounted from the first module without compromising the integrity of the dilution refrigerator.
We have realised that in contrast to known dilution refrigerator assemblies in which the sample has been located within the mixing chamber, it is possible to cool the sample located outside the mixing chamber (although typically within an evacuated chamber) and this then allows the assembly to be constructed from two modules. In particular, it is possible to provide all cooling services of the dilution refrigerator on or in the first module with experimental services only being provided by the second module. This means the second module need not be reliant on additional cooling power and allows second modules to be interchanged so as to enable many different classes of experiments to be made and also allows second modules or secondary inserts to be upgraded. For example, in the case of wiring for experiments, since the wiring is only attached to the second module, this allows different experiments to be performed by interchanging second modules.
By the phrase xe2x80x9cwithout compromising the integrity of the dilution refrigeratorxe2x80x9d we mean the structural and functional integrity although of course the dilution refrigerator may have to be removed from a surrounding cryostat to enable the second module to be dismounted.
The second module can be attached to the first module in a variety of ways including, for example, the use of clips, quick release bolts and the like but preferably the first module includes a series of thermal baffles defining aligned holes through which the second module can be inserted. This enables the second module to be easily attached to and demounted from the first module and also avoids the need for additional fixtures. The holes preferably have a diameter greater than 40 mm, typically about 50 mm.
Conveniently, in this case, the second module includes a number of thermal baffles, each aligned with a respective baffle of the first module when the two modules are attached. This provides an impedance to heat flow into the modules.
Preferably, the first and second modules are sealed together with a pair of spaced sealing assemblies. These may comprise an O-ring, for example in the form of a piston seal, and a pair of cooperating flanges held together using a clamp ring.
In some examples, the dilution refrigerator is mounted in a relatively non-detachable manner to the rest of the first module including 1K pot, still pumping line etc. Conveniently, however, the first module-includes a separate dilution unit comprising a still, heat exchanger and mixing chamber which can be demounted as a unit from the rest of the first module. This has the advantage of allowing different dilution units to be attached to the rest of the first module, for example having different performance or to allow upgrades. This is advantageous for example when different types of experiments are to be performed having specific performance requirements of the dilution refrigerator.
In most cases, the sample will be located in use within a magnetic field and typically the sample will be laterally offset from the axis of the second module. However, in the preferred example, the second module includes a sample holder which can be moved laterally with respect to the axis of the second module. Thus, while the second module is attached to the first module, the sample holder can be located in a first position and is thereafter moved laterally to a second position to bring it into alignment with a required part of a magnetic field, for example the centre of the magnetic field. Typically, the sample holder will be mounted via a pivot or slide connection to the rest of the second module.
In most applications the assembly will be located in use within a cryostat but alternatively could be cooled using a dry cooler such as a cryocooler.